Predicted standard deviation with simple linear regression


I am working on a prediction of wind speed. I'm using two datasets, one with 10 years of data, one with 1 year. I perform a simple linear regression for the dataset for the corresponding year. After I found the slope and offset i use the long dataset to make a prediction. The prediction of the mean is very good but the predicted standard deviation is lower. When i look into papers it seems that this is logic but nobody explains why. Can somebody help me with this,

Thanks in advance